In semiconductor integrated-circuit technology, metals are commonly used in the form of patterned layers for establishing electrical connections to and between individual devices such as, e.g., field effect transistors on a silicon chip or substrate; metal may be deposited over a dielectric which previously has been patterned for the sake of access to selected portions of semiconductor material. Typically, a free surface is blanketed with metal, and the deposited metal layer is then patterned to form the desired interconnection configuration. At present, aluminum is the material most widely used for integrated-circuit metallization; however, other refractory metals are receiving attention, and tungsten in particular. Blanket deposition of metal can be carried out, e.g., by (low-pressure) chemical vapor deposition, and patterning by conventional lithographic and plasma- or sputter-etching techniques.
Accurate pattern definition on a surface to be pattern-etched depends on adequate surface smoothness. However, chemical-vapor deposited metal often is found to have a relatively rough surface, surface roughness being attributed to undesirably large grain size in layers having desired thickness. Large grains also inhibit pattern definition in that, when the dimensions of features in a desired pattern become comparable to the grain size, adequate definition becomes difficult, if not impossible. Accordingly, and since preferred film thickness is determined primarily by the requirement that a deposited film have sufficiently high conductivity, it is desirable to deposit relatively thick layers which also have relatively small grain size.
According to one proposed method for producing fine-grained metal deposits, an intended metal layer is provided with grain-growth-interrupting sublayers of a metal compound; see U.S. Pat. No. 4,726,983, issued Feb. 23, 1988 to H. Hirada et al. For example, in the deposition of an aluminum layer by physical sputter deposition, periodic introduction of oxygen leads to the formation of sublayers of aluminum oxide. It is apparent, however, that the presence of compounds such as aluminum oxide, nitride, or carbide in a metallization layer tends to reduce conductivity. Reduced conductivity has also been observed in layers made by a method disclosed in U.S. Pat. No. 4,751,101, issued Jun. 14, 1988 to R. V. Joshi, where tungsten is deposited by silicon reduction of tungsten hexafluoride.